Who would have thought that it will be water that will boil the fish?
This proverb means that the thing which kills a man is part of him. A fish as used in this context has spent his whole life in water and must remain in water to be able to live. Still, water is used to boil the fish. When the crab gets into hot water, it gets to understand this tale- there are two types of water- that he will know. It is not bad to have, but having to the extent at which that which you have, have taken holds of you; is being in the realm of imprisonment.